Control circuit with high voltage starting means



n 1967, J. A. NUCKOLLS CONTROL CIRCUIT WITH HIGH VOLTAGE STARTING MEANS Filed Jan. 4, 1966 5 Sheets-Sheet 1 June 27, 1967 1 J. A. NUCKOLLS 3,328,673

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CONTROL CIRCUIT WITH HIGH VOLTAGE STARTING MEANS Filed Jan. 4, 1966 3 Sheets-Sheet 3 F g.4 T v United States Patent 3,328,673 CONTROL CIRCUIT WITH HIGH VOLTAGE STARTING MEANS Joe A. Nuckolls, Hendersonville, N.C., assignor to General Electric Company, a corporation of New York Filed Jan. 4, 1966, Ser. No. 518,618 16 Claims. (Cl. 32321) The present invention is a continuation-in-part of copending applications Ser. No. 223,480 filed Sept. 13, 1962, now US. Patent 3,249,807 dated May 3, 1966, Ser. No. 451,508 filed Apr. 28, 1965, and Ser. No. 458,353 filed May 24, 1965, all assigned to the same assignee as the present invention.

The present invention relates to control circuits for operating load devices, and more particularly concerns alternating current, phase-controlled circuits which employ controlled rectifier switching devices as disclosed in the aforementioned co-pending applications and which incorporate circuit means for automatically starting, operating and regulating the operation of load devices, such as gas discharge lamps.

It is an object of the invention to provide an improved control circuit of the above type which incorporates high voltage starting means for starting gas discharge lamps, such as sodium vapor lamps, or other lamp or load devices requiring a high voltage for starting.

It is a particular object of the invention to provide a control circuit of the above type which super-imposes a high voltage ignition pulse in the kilovolt range on the normal open circuit wave form of the circuit to ignite high starting voltage gas discharge lamps.

It is another object of the invention to provide a control circuit of the described type which may be employed for starting and operating load devices requiring either high or low voltage for starting.

Another object of the invention is to provide high voltage starting control circuits of the above type wherein the high voltage generation is de-energized automatically upon ignition of the load device.

It is still another object of the invention to provide a control circuit of the described type which provides a number of controlled starting and operating parameters for the lamp, such as optimum placement of the high voltage pulse relative to the half-cycle voltage wave form, lamp current limiting upon ignition, automatic forced lamp wattage buildup to the desired level, and controlled low current starting for certain lamps which have lower starting current than operating current.

Still another object of the invention is the provision of a control circuit of the above type incorporating feedback means to effect lamp current regulation when stabilized, and other forms of feedback.

Other objects and advantages will be apparent from the following description and the appended claims.

With the above objects in view, the present invention relates to a circuit for controlling the power applied to load means such as gas discharge lamps comprising, in combination, a source of alternating current, load means energized by the alternating current source, controlled rectifier means connected between the alternating current source and the load means, the controlled rectifier means being normally non-conductive to block current flow to the load means and having electrode control means to render it conductive, actuating means connected to the alternating current source and to the electrode control means for applying a control signal to the electrode control means at a predetermined time in each alternating current cycle, the actuating means including a resistance and a capacitance connected together in series and voltage sensitive symmetrical switch means connected across the 3,328,673 Patented June 27, 1967 capacitance, and means connected between the controlled rectifier means and the load means for providing a high voltage starting pulse on the load means.

In a preferred embodiment, the circuit includes a charging capacitor for providing an intermediate voltage higher than the source voltage and a high voltage pulse generating circuit in parallel with the charging capacitor and including another capacitor, a pulse transformer and a voltage sensitive switch device, the pulse generating circuit forming a high voltage, high frequency discharge loop for applying high voltage pulses in the kilovolt range to the load means for starting the latter.

The invention will be better understood from the following description taken in conjunction with the accompanying drawings, in which:

FIGURE 1 is a circuit diagram of an embodiment of a control circuit having a high voltage lamp starting means in accordance with the invention;

FIGURE 2 is a circuit diagram showing a portion of the control circuit having a dilferent embodiment of the high voltage starting means;

FIGURE 3 is a circuit diagram showing another embodiment of the control circuit of the invention having a modified high voltage lamp starting circuit; and

FIGURE 4 is a circuit diagram of another embodiment of the control circuit of the invention including feedback means for controlling the starting current of the load.

Referring now to the drawing, and particularly to FIG- URE 1, there is shown a phase controlled switching circuit for controlling the current and voltage applied to a load 1, such as a sodium vapor lamp or other variable impedance loads, connected to terminals 2 of a source of alternating current, typically 220 volts, by conductors 3 and 4. Ballast inductive reactance 6 is connected in series with lamp 1 to provide a current limiting impedance, as is conventional in discharge lamp circuits. Arranged in series with lamp 1 is a controlled rectifier circuit 5 which includes a paralleled pair of oppositely poled controlled rectifiers 7 and 8, which are typically silicon controlled rectifiers (SCR), having control (gate) electrodes 7' and 8 by means of which the SCRs are rendered conductive for unidirectional flow of current when a signal impulse is applied to the respective control electrodes.

Control electrodes 7' and 8' are connected to secondary windings 9a, 9b of coupling transformer 9. Transformer 9, which serves to isolate the controlled rectifier circuit 5 from the trigger pulse generating circuit, described below, is a pulse transformer which responds only to high frequency pulses, and therefore only high frequency trigger pulses are applied to the controlled rectifier circuit 5. This protects the control electrodes from over dissipation caused by follow-through current from the alternating current supply 2 appearing in the signal generating circuit which could otherwise tend to cause overheating and early failure of the controlled rectifier circuit.

The signal generating or actuating circuit 23 comprises a variable resistance 10 in series with a charging capacitor 11 connected across terminals 2, thereby synchronizing the signal generating function with the source voltage. A discharge loop in actuating circuit 23 for discharging capacitor 11 includes transformer primary 9c and a voltage sensitive device 12, typically a neon glow lamp, which is a bilaterally conducting gas tube and is also referred to herein as a voltage sensitive symmetrical switch means, which becomes conductive only upon the application of a predetermined voltage thereto. Glow lamp 12 can be replaced by a semiconductor voltage sensitive switch device having similar breakdown characteristics and pulsing capabilities, such device comprising, for example, a symmetrical Shockley device. Glow lamp 12 is connected to the source in parallel with capacitor 11 but is effectively connected in series discharge relation thereto, as shown, and with transformer primary 9c.

In the above described circuit arrangement, on each half cycle of the alternating current input, one of the controlled rectifiers 7 and 8 will have a positive anode and the other a positive cathode. Therefore, a control signal applied to control electrodes 7 and 8' will place only one of the controlled rectifiers in a conduction mode on each half cycle. A delay in the point in the alternating current input cycle at which the control signal impulse is applied to render the rectifier conductive is known as phase control.

As more fully described in the aforementioned copending applications, the disclosures of which are incorporated herein by reference, when glow lamp 12 becomes conductive as a result of voltage buildup on capacitor 11, capacitor 11 partially discharges and a signal pulse is applied to the transformer primary 9c which induces a current pulse of a particular duration and at a particular time in the half cycle. The controlled rectifier 7 or 8 which has an anode positive with respect to its cathode will then be triggered into conduction by the pulse current applied to control electrodes 7, 8' and the voltage which has built up across the rectifier falls substantially to zero. The controlled rectifier 7 or 8 then permits current to flow, consequently applying power to the load, for the remainder of that half cycle. On the next half cycle as the anode voltage becomes negative, the controlled rectifier 7 or 8 which was conductive becomes non-conductive and no power is transferred to the load until the signal generating circuit fires the other controlled rectifier. The time in the half cycle at which the rectifier is gated is adjustable by the level of resistance 10.

For the purpose of protecting the rectifier circuit from transient voltages, a thyrector 16, or double zener diode device, may be connected in parallel with the rectifiers, as shown.

The control circuit also preferably incorporates an integrating network 17 comprising series connected resistor 18 and capacitor 19 connected as shown across reactor 6, and variable resistor 10 is connected to network 17 at the junction of resistor 18 and capacitor 19. The characteristics and function of integrating network 17 are more fully disclosed in the aforementioned co-pending ap plication Ser. No. 458,353.

Capacitor 22 connected across source terminals 2 provides power factor improvement in the circuit. Capacitor 20, connected in parallel with lamp 1, is a voltage ringup capacitor and provides a maximum load impedance for the controlled rectifier circuit and serves to raise the voltage of the alternating current source to an intermediate level for purposes of starting the lamp, as explained hereinafter.

In accordance with the present invention, the circuit described incorporates a circuit to provide a high voltage pulse, e.g., in the range of 140 kilovolts or higher, for igniting gas discharge lamp 1 which may be a sodium vapor lamp or other vapor lamp requiring such high voltage for starting. The high voltage pulse generator comprises capacitor 60 and resistor 61 connected in series across capacitor 20, and step-up pulse transformer 62 connected in series between SCR circuit 5 and lamp load 1. The pulse generating circuit also includes a voltage sensitive symmetrical switch 63 such as a neon glow lamp connected across capacitor 60 and the primary winding of pulse transformer 62 and has a voltage breakdown level of a magnitude less than the peak voltage across capacitor 20. Typically, glow lamp 63 breaks down and becomes conductive at about 400 volts, whereas the peak voltage across capacitor 20 is about 600 volts. Glow lamp 63, capacitor 60 and the primary of transformer 62 constitute a resonant high frequency discharge circuit upon conduction through glow lamp 63, and in a typical case A oscillates at a frequency of kilocycles per second, providing a pulse of 5 kv. having a duration of 5 microseconds.

Step-up pulse transformer 62 typically has a primaryto-secondary turns ratio of 1 to 20, and is preferably but not necessarily an autotransformer as shown. The magnitude of the primary inductance and the turns ratio of transformer 62 and the magnitude of capacitor 60 can be selected to provide various peak ignition voltages, durations, and energy levels as desired. Also the magnitude of resistor 61 can be selected or adjusted relative to the magnitude of capacitor 60 to adjust the placement of the ignition pulse with respect to the half-cycle voltage.

In the operation of the described circuit, upon actuation of SCR circuit 5 as described, an intermediate voltage charge (e.g., 600 volts) is placed on capacitor 20 by virtue of the resonant charging circuit formed by series connected inductance 6 and capacitor 20. While such resonant charging effect makes possible a very high voltage generating capability across capacitor 20, the voltage clamping action of thyrector 16 limits the voltage buildup in capacitor 20.

Capacitor 60 charges up toward the peak voltage appearing across capacitor 20, e.g., 600 volts, and at the voltage breakdown level of glow lamp 63, e.g., 400 volts, the latter becomes conductive, placing the voltage of capacitor 60 in a pulse across the primary of transformer 62. With the step-up ratio of transformer 62 being 1:20, a high frequency pulse of about 8 kilovolts is applied from the secondary of the transformer to lamp 1. The negative coefficient of resistance of lamp 1 at starting causes the arc drop across the lamp to fall to a low voltage level and the voltage across capacitor 20 no longer builds up as a result of lamp loading. Therefore, the voltage across capacitor 60 can no longer reach the breakdown voltage of switching device 63, and the operation of the ignition pulse generating circuit is automatically discontinued when lamp 1 starts. When a lamp outage occurs, the starting mechanism is automatically re-applied.

A minimum capacitance in power factor capacitor 22 is often required to provide a stiff instantaneous source of current to ensure the high voltage ring-up across capacitor 20. Therefore, capacitor 22 should have adequate highfrequency capabilities.

The circuit shown in FIGURE 1 includes a current feedback system as also disclosed in the aforementioned copending application Ser. No. 458,353. The feedback system comprises a feedback circuit 33 connected across reactor 6 and including variable resistor 32 and incandescent lamp 30. Employed in conjunction with incandescent lamp 30 is a photosensitive circuit 35 including photoconductor 15, such as a cadmium sulfide cell, arranged seeing lamp 30, and in series with capacitor 36. Circuit 35 is connected at one end to integrating network 17 at the junction of capacitor 19 and resistor 18, and at the other end in series with capacitor 13. The feedback system provides lamp load wattage regulation compensating for line voltage variations and to a large degree for lamp voltage variations, and essentialy effects constant current. This feedback system is more fully described in the last mentioned co-pending application.

FIGURE 2 shows a modified high-voltage pulse generating circuit which may alternatively be used in the control circuit shown in FIGURE 1. In the embodiment of FIGURE 2, voltage sensitive symmetrical switch 63a such as a spark tube or glow lamp in series with capacitor 60a is connected to the primary of autotransformer 62 and in parallel with charging capacitor 20 as shown. In this embodiment resistor 61 is omitted. This circuit serves to place the ignition pulse on the leading edge of the 60-cycle wave form occurring across capacitor 20, and functions as follows. As the voltage rises across capacitor 20, it also rises across spark tube 63a to the breakdown potential of the latter, which then conducts a current pulse from capacitor 20 to capacitor 60a, placing the instantaneous voltage of capacitor 20 across the primary of transformer 62 and thereby applying the ignition pulse to the leading edge of the voltage buildup occurring across capacitor 20 each half-cycle.

In the FIGURE 1 circuit, the pulse is always applied sometime after the leading edge as determined by the RC time constant of resistor 61 and capacitor 60. The earlier placement of the ignition pulse by the modification shown in FIGURE 2 provides earlier in-cycle ignition of the lamp load, thereby enabling a greater remaining portion of the wave half-cycle to be applied to the lamp following the high voltage ignition pulse. In the FIGURE 2 em bodiment the high frequency discharge circuit comprises capacitor 20, switch device 63a, and capacitor 60a connected in series with the primary of transformer 62. In this connection, symmetrical switch 63a upon becoming conductive applies the instantaneous sum voltage of capacitor 20 and the residual voltage of capacitor 60a to the primary turns of transformer 62. As the current pulse flows, an opposing voltage buildup occurs across capacitor 60a, thereby limiting the switch follow-through current.

In addition to this current limiting function, and the provision of the dominant capacitance involved in the resonant high frequency, high voltage generation which also characterize corresponding capacitor 60 in the FIG- URE 1 circuit, capacitor 60a serves in the FIGURE 2 embodiment to hold a residual charge aiding the next half cycle high voltage breakdown of spark tube 63a and operating to place the ignition pulse on the leading edge of the wave. The spark tube 63a can be replaced by a semiconductor device having similar breakdown characteristics.

FIGURE 3 shows a modification of the high voltage pulse generating circuit which provides an ignition pulse which is positive-going with respect to the voltage buildup of capacitor 20, in contrast to the negative-going ignition pulse provided by the FIGURE 2 arrangement. This feature may be preferred for use with certain types of lamps and circuitry. In this embodiment, charging capacitor 20a is connected to a tap of transformer 62 between its primary and secondary windings, and capacitor 60b in series with voltage sensitive symmetrical switch 63b is connected to the start of the primary winding of transformer 62 and forms therewith and with capacitor 20a a high frequency, high voltage discharge loop. The transformer tap connection is polarized as shown so as to provide a positive-going ignition pulse with respect to the voltage buildup across capacitor 20a. With this connection, as the voltage builds up across capacitor 20a, the voltage also appears across switch 63b. When the latter becomes conductive at its 'voltage breakdown level (e.g., 400 volts), the high frequency current flows from the positive terminal of capacitor 20a through the tap on transformer 62 to the start of its primary winding and through the series connected capacitor 60b, placing approximately 400 volts across the primary turns of transformer 62, the polarity being such that positive voltage appears at the tap and negative voltage at the primary start. Consequently, the high voltage side of pulse transformer 62 provides a positive-going pulse across lamp 1 with respect to the voltage appearing across capacitor 20. When the opposing voltage buildup across capacitor 60b reaches the de-ionization level for neon glow lamp 63b, the latter de-ionizes, thus turning off the high voltage pulse.

A variable resistance 64 may be placed in parallel with switch 63b to ensure its extinguishing over a large input voltage range. This variable resistance may be constituted by a thermistor as shown, which forces the high voltage pulse turnoff as a result of self-heating after a period of pulsing. This function may be desired for the purpose of turning off the ignition pulse if lamp 1 burns out.

Also shown in the circuit diagram of FIGURE 3 is an auxiliary photosensitive control circuit comprising photoconductor 40 connected across capacitor 48 and neon glow lamp 12 for automatically turning lamp 1 on and off at a desired ambient light level, as more fully disclosed in co-pending application Ser. No. 458,353. In addition, there is shown feedback circuit 73 including variable resistance 74 and incandescent lamp 72 for compensating for line voltage variation, as also disclosed in the last mentioned co-pending application. Variable resistance 74 should have a minimum resistance sufiicient to protect the incandescent lamp from excessive voltages.

Arranged adjacent incandescent lamp 72 is photoconductor 41 which in series with capacitor 48 is placed in parallel with actuating circuit 23. In the operation of this system, if the line voltage increases, the current through incandescent lamp 72 increases, brightening lamp 72 and causing photoconductor 41 to decrease in resistance and thereby shunting more of the available charging circuit from capacitor 11. This slows the voltage buildup across capacitor 11, hence delaying the firing of SCR circuit 5 and regulating the power delivered to lamp load 1.

The photoelectric control for turning the system on and off is achieved by exposing photoconductor 40 to the ambient light. As the ambient light level increases, the photocell resistance increases allowing the current to pass around neon glow lamp 12, keeping the latter from reaching a breakdown voltage level and becoming conductive, and thus preventing operation of the triggering or signal generating circuit 23. Photoconductor 41 can be used, if desired, as the ambient light sensing photoconductor in lieu of photoconductor 40 to turn off the actuating circuit. As more fully disclosed in the last mentioned co-pending application, capacitor 48 serves both as a symmetry forcing component during the photoelectric current on and current oif functions and provides more stabilized footcandle levels at which such svw'tching occurs, and also linearizes the normal feedback function. Damping resistor 54 is placed in series with capacitor 13 to ensure a surge or damped trigger pulse so as to provide a discharge without oscillatory tendencies or multiple pulsing.

Also shown in FIGURE 3 is a voltage sensitive switch 70 such as a neon glow lamp in series with resistance 71 and connected across incandescent lamp 72 for the purpose of turning off lamp 1 in the event of failure of feedback circuit 73 due to burning out of incandescent lamp 72, glow lamp 70 being arranged to illuminate photoconductor 41 for disabling the operation of the actuating circuit 23 upon failure of lamp 72.

Another feature shown in the FIGURE 3 circuit is the provision of a branch circuit comprising capacitor in series with a switch device 81 connected across capacitor 20a for the purpose of obtaining higher high-voltage starting energy where this is desired or necessary to effectively pick up or start certain lamp loads by applying the high voltage pulses at much higher energy levels than is afforded by the use of capacitor 20a alone. Switch device 81 may be a symmetrical voltage sensitive switching device such as the double Zener device shown, a thyrector, or back-to-back SCRs having a voltage sensing triggering means, or a double Shockley device, or it may be simply a manually operable switch used only for the starting operation. Capacitor 80 typically will have the same volt- ,age rating as capacitor 20a, and at the intermediate voltage level to which the latter becomes charged as previously described, capacitor 80 discharges its energy as switch device 81 becomes conductive at that voltage level and adds its start energy to that of capacitor 20a. When the lamp ignites, the voltage in the ignition circuit drops and the operation of the double Zener device forces the automatic dropout of capacitor 80. Such dropout is necessary since too high a capacitance during the normal operation of the lamp would cause excessive peak lamp current which would result in shortened lamp life.

Certain lamps such as sodium vapor lamps require during the initial lamp warm-up period a low lamp current which is less than the normal lamp operating current. FIGURE 4 shows a lamp control circuit which incorporates a feedback modification to provide such a function. In this embodiment, thermistor 85 or other suitable variable resistance is connected between integrating network 17 and actuating circuit 23 and placed in proximity to, i.e., in thermal contact with, heating resistor 86 connected in series between reactor 6 and SCR circuit 5. This feedback which, as shown, may function in conjunction with current feedback circuit 33 (which is more fully disclosed in co-pending application Ser. No. 458,353) operates as follows. At the instant of lamp starting, thermistor 85 and resistor 86 are at ambient temperature. The resistance of thermistor 85 is thus relatively high, providing a retarding of the SCR firing phase angle and thereby limiting the current applied to the lamp load 1. With lamp current flowing for a period of time, resistor 86 becomes heated thereby, in turn heating thermist-or 85. The times of heating involved can be adjusted by selection of the total thermal mass to be heated (e.g., of resistor 86 or associated mass), the resistance of resistor 86, or the type of thermistor employed. The increase in thermistor temperature causes a decrease in resistance therein to saturation level, which in turn advances the SCR firing angle, causing the lamp current to increase to some controlled level.

In the case where the type of lamp load used requires a higher starting current than operating current, the thermistor 85 could be of a type which increases in resistance with increase in temperature.

It will be understood that the circuit arrangements of the invention may find application for various types of loads other than gas discharge lamps, such as electric and electronic welding equipment, electric ignition systems, and other apparatus requiring high starting voltages.

While the present invention has been described with reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the scope of the invention. Therefore, the appended claims are intended to cover all such equivalent variations as come within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A circuit for controlling the power applied to load means comprising, in combination, a source of alternating current, load means energized by said alternating current source and requiring a high voltage starting pulse, controlled rectifier means connected between said alternating current source and said load means, said controlled rectifier means being normally non-conductive to block current fiow to said load means and having electrode control means to render it conductive, actuating means connected to said alternating current source and to said electrode control means for applying a control signal to said electrode control means at a predetermined time in each alternating current cycle, said actuating means including a resistance and a capacitance connected together in series and to the alternating current source and voltage sensitive symmetrical switch means connected across said capacitance, means connected between said alternating current source and said load means for producing an intermediate voltage higher than the voltage of said alternating current source, and means connected between said controlled rectifier means and said load means for providing a high voltage starting pulse on said load means.

2. A circuit as defined in claim 1, said intermediate voltage producing means including impedance means connected in series with said load means and said controlled rectifier means, a first capacitor connected across said load means and in series with said impedance means and forming therewith a resonant circuit, said high voltage pulse producing means comprising a second capacitor, voltage sensitive switch means and step-up pulse transformer means connected together in circuit across said first capacitor for producing on said load means repetitive pulses of a voltage substantially higher than said intermediate voltage for starting said load means.

3. A circuit as defined in claim 2, wherein said pulse transformer means has a primary winding and a secondary winding, and said second capacitor is connected in series with said primary winding in series with sald voltage sensitive switch means connected across the first capacitor and said secondary winding being connected in series with said load means.

4. A circuit as defined in claim 3, wherein a resistor is connected in series with said second capacitor across said first capacitor, said voltage sensitive switch means being connected to the junction of said second capacitor and said resistor and forming with said second capacitor and said primary winding a high frequency, high voltage discharge loop.

5. A circuit as defined in claim 3, wherein said pulse transformer means is an autotransforrner having a tap between said primary and secondary windings, and said series connected second capacitor and voltage sensitive switch means are connected to said tap, and said first capacitor is connected to the start of said primary wind- 6. A circuit as defined in claim 3, wherein said pulse transformer means is an autotransfor-mer having a tap between said primary and secondary windings connected to said controlled rectifier means, and said series connected second capacitor and voltage sensitive switch means are connected to the start of said primary winding, and said first capacitor is connected to said tap.

7. A circuit as defined in claim 3, wherein said first capacitor, said second capacitor, said voltage sensitive switch means and said primary Winding form a high frequency, high voltage discharge loop.

8. A circuit as defined in claim 7, wherein a variable resistance is connected across said voltage sensitive switch means for disabling the operation of the latter in the event of failure of said load means.

9. A circuit as defined in claim 8, wherein said variable resistance across said voltage sensitive switch means is a thermistor.

10. A circuit as defined in claim 3, wherein a third capacitor and second switching means are connected in series across said first capacitor for providing increased energy for starting said load means.

11. A circuit as defined in claim 10, wherein said switching means is a symmetrical voltage sensitive switching means.

12. A circuit as defined in claim 1, wherein a pair of line conductors connect said load means and said alternating current source, said circuit further including feedback circuit means comprising light producing means connected between said line conductors for detecting voltage variations across said conductors, photosensitive means responsive to said light producing means and connected to said actuating means for controlling the operation thereof in response to the light output of said light producing means, and a voltage sensitive light producing swit-ch device connected across said first mentioned light producing means and arranged to illuminate said photosensitive means for disabling the operation of said actuating means upon failure of said first mentioned light producing means.

13. A circuit as defined in claim 1, including feedback means connected to said alternating current source and said actuating means for producing during the load starting interval a load current different from the normal load operating current.

14. A circuit as defined in claim 13, wherein said feedback means comprises a heating resistance connected between said alternating current source and said load means, and a temperature sensitive variable resistance connected between said alternating current source and said actuating means and arranged in proximity to said heating resistance, and operating in response to variations in temperature of said heating resistance to control the operation of said actuating means and thereby control the load current.

15. A circuit as defined in claim 14, wherein said temperature sensitive variable resistance is a thermistor which decreases in resistance With a rise in temperature.

16. A circuit for controlling the power applied to load means comprising, in combination, a source of alternating current, load means energized by said alternating current source and requiring a high voltage starting pulse, controlled rectifier means connected between said alternating current source and said load means, said controlled rectifier means being normally non-conductive to block current flow to said load means and having electrode control means to render it conductive, actuating means connected to said alternating current source and to said electrode control means for applying a control signal to said electrode control means at a predetermined time in each alternating current cycle, said actuating means including a resistance and a capacitance connected together in series and to the alternating current source, and voltage sensitive symmetrical switch means connected across said capacitance, and means connected between said controlled rectifier means and said load means for providing a high voltage starting pulse on said load means, said high voltage pulse producing means comprising a capacitor, voltage sensitive switch means and stepup pulse transformer means connected together in circuit and between said controlled rectifier means and said load means.

References Cited UNITED STATES PATENTS 3,120,620 4/1964 Nowell 307-885 3,130,347 4/1964 Harpley 315-198 x 3,170,085 2/1965 Genuit 315-273 x 3,171,040 2/1965 Goebel 307-885 3,180,974 4/1965 Darling 323-22 3,189,790 6/1965 Nuckolls 315-289 3,249,807 5/1966 Nuckolls 315 199 JOHN F. COUCH, Primary Examiner. W. E. RAY, A. D. PELLINEN, Assistant Examiners. 

16. A CIRCUIT FOR CONTROLLING THE POWER APPLIED TO LOAD MEANS COMPRISING, IN COMBINATION, A SOURCE OF ALTERNATING CURRENT, LOAD MEANS ENERGIZED BY SAID ALTERNATING CURRENT SOURCE AND REQUIRING A HIGH VOLTAGE STARTING PULSE, CONTROLLED RECTIFIER MEANS CONNECTED BETWEEN SAID ALTERNATING CURRENT SOURCE AND SAID LOAD MEANS, SAID CONTROLLED RECTIFIER MEANS BEING NORMALLY NON-CONDUCTIVE TO BLOCK CURRENT FLOW TO SAID LOAD MEANS AND HAVING ELECTRODE CONTROL MEANS TO RENDER IT CONDUCTIVE, ACTUATING MEANS CONNECTED TO SAID ALTERNATING CURRENT SOURCE AND TO SAID ELECTRODE CONTROL MEANS FOR APPLYING A CONTROL SIGNAL TO SAID ELECTRODE CONTROL MEANS AT A PREDETERMINED TIME IN EACH ALTERNATING CURRENT CYCLE, SAID ACTUATING MEANS INCLUDING A RESISTANCE AND A CAPACITANCE CONNECTED TOGETHER IN SERIES AND TO THE ALTERNATING CURRENT SOURCE, AND VOLTAGE, SENSITIVE SYMMETRICAL SWITCH MEANS CONNECT- 